As shown in FIG. 1, which shows one embodiment of the present invention, a tool holder A used to mount a cutting tool on a machining center includes a shank portion 1 which can be detachably mounted to a spindle of a machining center, and a chuck portion 2 fixed to the shank portion 1 through a grip portion 3 to be gripped by a manipulator.
Since today's spindles are rotated at increasingly high speeds, it is for example required that tool holders be capable of gripping a cutting tool 30 strongly and with high accuracy, have sufficient rigidity, rotatable in a balanced manner. Among tool holders that meet these requirements are known as shrink fit tool holders, in which the cutting tool 30 is fitted in the chuck portion 2 by shrink fitting (which are disclosed in the below-identified Patent documents 1 to 4).
Due to recent advanced technology, increasing numbers of parts made of materials which are difficult to cut, such as aluminum alloys and titanium alloys, are used in aerospace, motor-related and other industries. These parts include aluminum structural parts such as airframes, wing ribs and bulkheads of aircraft, titanium structural parts such as air frame components and edge frame, and blades and impellers of motors.
Spindles of machine tools (machining centers) used to efficiently machine or cut these difficult-to-cut materials are rotated at as high as 33000 rpm or over, with the output of the spindle motors at as high as 80 kW (100 HP) or over, in machining aluminum, and the spindle torque could reach 1500 Nm in machining titanium. Machining centers capable of withstanding such high loads are now commercially available.
Since conventional machining centers having three axes (capable of moving up and down, right and left, and back and forth) are low in machining efficiency, 5-axis control machining centers, which have two rotation center axes (A- and B-axes) in addition to the above three axes, are now increasingly frequently used.
During machining of such difficult-to-cut materials, especially using a 5-axis control machining center, especially high gripping force (gripping torque) is required for the cutting tool holder.
The gripping force is the force which receives and withstands cutting loads applied to the cutting tool 30 during cutting. The larger this gripping force, the higher the cutting efficiency. In other words, the larger the gripping force (gripping torque) of the tool holder, the higher the cutting ability is, and the smaller the gripping force, the lower the cutting ability. For higher efficiency, it is also required that the tool holder have high rigidity (due e.g. to a large diameter of its grip portion, and its short effective length) and the cutting tool also have high rigidity (due e.g. to a short length of its protruding portion and due to the fact that it is made of cemented carbide). The gripping force is important because the larger the gripping force, the deeper it is possible to cut the material at a higher feed rate, which means that the higher the gripping force, the higher the machining efficiency, which significantly influences productivity and costs.
When machining aluminum, the higher the cutting speed is, the higher the cutting efficiency tends to be. Since aluminum is lower in strength than steel, it can be cut deeper. Thus, when machining aluminum, the depth of cut is set at a larger value. But if the depth of cut is large, the tool holder and the cutting tool tend to chatter (vibrate) violently. Violent chattering (vibration) of the tool holder and the cutting tool could easily cause slippage between the tool holder and the cutting tool, even if the gripping force is considered to be sufficient. Such slippage could result in the cutting tool being pulled out of the tool holder.
On the other hand, titanium is high in mechanical strength, low in thermal conductivity (and thus heat dissipation), high in affinity for other metals, and can easily react with oxygen and nitrogen, so that titanium is classified as a difficult-to-cut material. Titanium generates peculiar serration-shaped chips due to fluctuations in cutting force. Due to fluctuations in cutting force, violent “chattering” occurs, which causes premature wear of the cutting tool. The worn cutting tool increases cutting torque, which could in turn causes slippage of the cutting tool or the cutting tool to be pulled out of the tool holder.
Parts for aircraft are frequently manufactured by cutting a huge material to be cut. Such material to be cut is expensive and if parts are excessively cut due to separation of the cutting tool, it is impossible to repair such excessively cut parts, resulting in immense loss.
Thus, it is an ordinary practice to increase safety standards and reduce the standards for machining conditions. But this makes it impossible to perform machining with high efficiency expected for such machining.
As explained above, since shrink fit tool holders can cope with high-speed rotation of spindles, can grip the cutting tool 30 with high accuracy and with high strength, are rigid, and allows the cutting tool to be rotated in a balanced manner, slender and tapered shrink fit holders are frequently used in the mold manufacturing field, especially during deep carving. This is mainly because by using such a holder, the holder is less likely to interfere with the workpiece, so that it is possible to use a relatively short cutting tool, which increases the rigidity of the cutting tool, thereby improving the machining accuracy and increasing the life of the cutting tool (see Patent document 1).
But in the fields of high-efficiency machining of aluminum and machining of difficult-to-cut materials such as titanium, including the aircraft-related fields, such tapered shrink fit holders are not considered to be as satisfactory as used in the mold manufacturing field.